The design and operation of a pulverized coal fired boiler is more dependent upon the effect of mineral matter in the coal than any other single fuel property. The sizing of the boiler and its design are largely determined by the behavior of the coal mineral matter as it forms deposits on the heat transfer surfaces in the lower furnace. Operation of the boiler may be affectd by the thermal, physical and chemical properties of the deposits. Ash deposits on the heat transfer surfaces can inhibit the heat absorption rates and with some coals can also cause corrosion of the heat transfer surfaces.
Another very important consideration in pulverized coal firing of steam generators is the production of nitrogen oxides (NO.sub.x). Regulatory standards limiting the extent of NO.sub.x production from steam generators are being increasingly stringent in order to protect our environment. A variety of techniques to control NO.sub.x via combustion modifications have been studied by researchers throughout the world and it is very likely that the design of future fuel firing systems for steam generators will be greatly affectd by the stringency of regulatory standards and the available control techniques.
The transformation of mineral matter and the formation of NO.sub.x during combustion of pulverized coal are very complex phenomena involving aero-dynamics, physical, chemical and thermal considerations. Mineral matter in coal varies in composition and properties depending on the type of coal and its geographical origin. Laboratory research reveals that iron compounds comprise some of the key constituents in coal mineral matter relative to their contribution to the phenomena of slag formation. Slag formation on furnace walls can occur because of selective deposition of low-melting ash constituents. These low-melting ash constituents melt within the furnace into spherical globules that, due to their low drag coefficient, do not follow gas streamliners, and are deposited on the furnace walls. In conventional tangential fired systems, due to the inherent aero-dymanics, a reducing or low-oxygen atmosphere can occur in localized zones adjacent to the water-wall tube surfaces. Furthermore, it is an established fact that iron compounds of the type found in ash deposits have a lower melting point in a reducing atmosphere. The conventional firing system can result in slagging by a combination of localized reducing atmosphere in the vicinity of lower furnace walls and the selective deposition of low-melting constituents because of their inability to follow gas streamliners.
The phenomenon of NO.sub.x formation in pulverized coal-fired furnaces is also quite complex. The extent of NO.sub.x formation depends on the type of coal, furnace firing rate, mixing conditions, heat transfer, and chemical kinetics. Two major forms of NO.sub.x have been recognized; thermal NO.sub.x and fuel NO.sub.x. Thermal NO.sub.x results from the reaction of nitrogen in the air with oxygen and is highly temperature dependent. In a typical tangentially fired furnace using pulverized coal, the contribution of thermal NO.sub.x to the total NO.sub.x is less than about 20%, due to relatively low temperatures throughout the furnace. The present invention will not adversely affect this advantage with respect to thermal NO.sub.x.
The major contributor of NO.sub.x is the fuel NO.sub.x, which results from the reaction of fuel nitrogen species with oxygen. The fuel NO.sub.x formation is not very highly temperature dependent, but is a strong function of the fuel-air stoichiometry and residence time. A number of techniques to control fuel NO.sub.x have been developed to date, that involve modification of the combustion process. Some of the important ones involve low-excess-air firing and air staging.
A third form of NO.sub.x, known as prompt NO.sub.x, has also been recognized by researchers. Prompt NO.sub.x results from the combination of molecular nitrogen with hydrocarbon radicals in the reaction zone of fuel-rich flames. Formation of both the fuel NO.sub.x and prompt NO.sub.x involves intermediates such as CN, NH, and other complex species.
In pulverized coal firing, fuel nitrogen is evolved during both the devolatization and char burn-out stages. The degree of fuel nitrogen evolution during devolatization is a function of temperature and heating rate of coal particles. Further, the degree of conversion of evolved fuel nitrogen into NO.sub.x is highly dependent on the stoichiometry and residence time. Under fuel-rich conditions and with sufficient residence time available, the conversion of fuel nitrogen to harmless molecular nitrogen, rather than to NO.sub.x, can be maximized.
In present-day tangentially fired systems, although the coal jet injected into the furnace of fuel-rich, the residence time available for conversion of volatile nitrogen to molecular nitrogen is extremely short before the jet contacts the oxygen-rich body of the tangential vortex. Further, the auxiliary air jets adjacent to the fuel-rich coal jet may interact with the nitrogen intermediates to yield NO.sub.x at the interface.